دو ماهنامه علوم و تکنولوژی پلیمر
سال سی‌ام شماره 3 (پیاپی 149، امرداد و شهریور 1396)

In recent years, different color indicators have been developed and utilized in smart packaging to better visualize the fruit shelf-life and its safety consumption date as well as to minimize the loss of agricultural products. In this study, a potassium permanganate solution-containing polyethylene nanocomposite film was prepared through melt mixing process as color indicator for smart packaging. Two kinds of silica nanoparticles of different surface hydrophobicity were incorporated in the LDPE films to study the effect of hydrophilicity of nanoparticles on the film barrier properties. The morphology and dispersion of nanoparticles were studied using SEM/EDX technique. The gas permeability, dynamic scanning calorimetry, melt flow index and mechanical properties were investigated to find an optimum formulation. The results of the oxygen barrier tests showed that the increase of nanoparticles loading in the polymer matrix increased the permeability up to 95% for the sample containing 5% hydrophilic silica. The hydrophilic nanosilica was well dispersed in the matrix and generated void channels which allowed to form a permeable polymer film. The presence of nanosilica did not alter the polymer crystallinity as well as the mechanical properties of the nanocomposite films. The melt flow index data showed that the silica/polyethylene nanocomposites could be produced with appropriate processability. The color indicator was then fabricated using potassium permanganate placed on a woven fabric. The whole colored fabric was then put within a sealed permeable polyethylene bag. The efficiency of the color indicator against ethylene gas has been measured for a duration of 10 days which is suitable in kiwi fruit packaging.

The effects of process conditions and their interactions on the catalyst activity in 1-hexene polymerization were studied with design of experiments by response surface methodology (RSM) using a commercial Ziegler-Natta (ZN) catalyst in the form of TiCl4/MgCl2/Di-n-butyl phthalate. The effect of different operational variables on the catalyst activity was examined by performing the primary experiments of 1-hexene polymerization. Among different operational variables, three variables including monomer concentration, polymerization temperature and cocatalyst/catalyst molar ratio (Al/Ti) were considered as the main parameters which affected the catalyst activity in the 1-hexene polymerization. The Box-Behnken model with three main parameters in three level responses for each factor was applied to analyze the parameter relationships. After demonstrating the reproducibility of the experimental results, the statistical analysis of experimental data showed that the monomer concentration and Al/Ti molar ratio affected the catalyst activity significantly. It was found that, at room temperature, by increasing the monomer concentration from 80.0 mmol to 239.9 mmol, the activity of the studied ZN catalyst increased from 75.2 to 265.1 gpoly(1-hexene)/gcat. In addition, by changing the Al/Ti ratio from 45.9 to 136.8, the catalyst activity increased from 145.2 to 265.1 gpoly(1-hexene)/gcat. The maximum activity of catalyst was obtained at the polymerization temperature around 25°C, and by increasing the temperature the activity of studied catalyst decreased. Based on the RSM, the best polymerization condition was obtained at a polymerization temperature of about 35°C, Al/Ti ratio of 136.8, and monomer concentration of 239.9 mmol, which resulted in maximum productivity of the catalyst.

Carbon nanotubes (CNTs) are a new class of nanomaterials that have gained special attention due to their unique properties such as excellent electrical and mechanical properties. Nanocomposite hydrogels, a novel category of hydrogels, have received great attention both in industry and scientific research because of their exceptional structural and mechanical properties. A nanocomposite aqueous dispersion based on poly(acrylamide-co-acrylic acid) and modified carbon nanotube was synthesized through in situ radical polymerization. Water can be a good candidate instead of toxic organic solvents for preparation of poly(AA-co-AM)/CNT nanocomposite aqueous dispersions. The rheological properties of the nanocomposites were significantly improved compared to those of pure copolymer samples. Modification of carbon nanotubes by acid was conducted to introduce hydroxyl and carboxyl groups on their surface in order to achieve a better dispersion behavior and suitable interactions between the nanoparticles and polymer matrix. Once the oxidation step was finished, amide functional groups were inserted into the CNT particles through amidation reaction. The surface modification reactions of CNT were tracked by FTIR and Raman spectroscopy techniques. FTIR and Raman spectra were utilized in order to investigate the dispersion behavior of nanoparticles and to confirm the formation of linkages between the nanoparticles and polymer matrix, respectively. In addition, the rheological features including viscoelastic behavior of samples, the sol-gel transition phenomenon, dynamic oscillatory frequency sweep and steady shear measurements were studied. Finally, the relationship between the improved rheological properties (modulus and viscosity) and the dispersion microstructures caused by dispersion of nanoparticles, formation of networks and interfacial interactions between Poly(AA-co-AM) macromolecular chains and CNT nanoparticles were determined.

Different macromonomers having polyethylene glycol branches were synthesized via esterification reactions in various conditions. Quaternary polymers were prepared using synthesized macromonomers of polyoxyethylene acrylate and methacrylate with a PEG molecular weight of 600-3000 g/mol, and sodium acrylate, sodium methacrylate and sodium maleate. The superplasticizers were synthesized by free radical polymerization in water medium at 65-80oC. The recipe and polymerization conditions have a direct effect on the structure of superplasticizers. The structures of the synthesized quaternary polymers were characterized by 1H NMR, Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) analyses. The efficiency of a superplasticizer depends on the size of main chain, the chemical structure of repeating unit, molar ratio of monomers, chain-to-ion molar ratio (B/I) and size of the branches. These structural parameters affect the geometrical restriction, adsorption and the interaction between the superplasticizer chains and cement particles. FTIR, thermal gravimetric analysis (TGA) and X-ray powder diffraction (XRD) methods were also used to characterize the effect of superplasticizer structure on the hydration reaction of the cement pastes. FTIR spectroscopy was used to explore the effect of superplasticizer structure such as branch length and chain-to-ion molar ratios on the structure of the hydrated calcium silicate gels, polysilicate (SiO4-2) and calcium hydroxide generated during 7 days hydration. The results of XRD indicated that the structure of superplasticizer affects the content of anhydrous phase and hydrated products during hydration. The increase of PEG branch length slightly increased the decomposition temperatures of the hydrated calcium silicate gels and calcium hydroxide crystals. Furthermore, the size of calcium hydroxide crystals changed with the superplasticizer branch size.

Sandwich structures are widely used in aerospace, automobile, high speed train and civil applications. Sandwich structures consist of two thin and stiff skins and a thick and light weight core. In this study, the obligatory mandate of a sandwich plate contact constitutes a flexible foam core and composite skins with a hemispherical rigid punch has been studied by an analytical/empirical method. In sandwich structures, calculation of force distribution under the punch nose is complicated, because the core is flexible and the difference between the modulus of elasticity of skin and core is large. In the present study, an exponential correlation between the contact force and indentation is proposed. The coefficient and numerical exponent were calculated using the experimental indentation results. A model based on a high-order sandwich panel theory was used to study the bending behavior of sandwich plate under hemispherical punch load. In the first method, the force distribution under the punch nose was calculated by the proposed method and multiplied to deformation of related point in the loading area to calculate the potential energy of the external loads. In the second method, the punch load was modeled as a point force and multiplied to deformation of maximum indented point. The results obtained from the two methods were compared with the experimental results. Indentation and bending tests were carried out on sandwich plates with glass/epoxy skins and a styrene/acrylonitrile foam core. In the bending test, a simply support condition was set and in the indentation test the sandwich specimens were put on a rigid support. Indeed, in this position the punch movement was equal the indentation. The comparison between the analytical and experimental results showed that the proposed method significantly improved the accuracy of analysis.

Because of wide applications of poly(ethylene oxide) (PEO) in various areas such as chemical, electrical and pharmaceutical industries, researchers have been focused to develop and improve the properties of this interesting polymer. To improve the mechanical and thermal properties of PEO some research works have been done. One way to improve the properties of this polymer is to add natural nanoparticles and producing the corresponding nanocomposites. In this research, a poly(ethylene oxide)/chitin nanofibrils (CFNs) nanocomposite was prepared at different CNFs loadings, and the effect of nanochitin on the properties of nanocomposite was investigated by different techniques. Nanochitin was prepared using a 1 wt% chitin suspension in water by a mechanical super-grinder, and the SEM images showed an average 50 nm diameter for the fibers obtained. Poly(ethylene oxide)/nanochitin (PEO/NFC) nanocomposites having 1, 3 and 5 wt% NFC were prepared via solution casting method using water as solvent. The dynamical mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA) results showed that the storage modulus and degradation temperature of the nanocomposites increased with NFC loading. The SEM images showed a considerable difference in the morphology of specimens. The X-ray diffraction (XRD) patterns revealed a remarkable reduction in crystallinity of the semi-crystalline PEO. The results of X-ray diffraction tests showed that at low loadings of nanochitin there was no peak in XRD diffractograms. The TGA results showed that with the addition of nanochitin to poly(ethylene oxide) the degradation temperature increased. The DMTA results showed an increase in the storage modulus (G') of the nanocomposites by increasing nanochitin loading. The maximum improvement in thermal and mechanical properties was observed at 5 wt% CFN loading.

Bitumen as a really important material road construction shows very attractive properties. Modification of bitumen by polymers increases the service life of the asphalt pavement. In order to improve the engineering properties of bitumen, both chemical and physical methods have been used which the physical methods, because of their simplicity, are more commonly employed. In physical modification, some materials such as polymers, are usually used. Polysulfide is one of the polymers that has recently attracted the interest of industries. It is interesting that this polymer which is made of heavy chlorinated petrochemical was tested to reduce the costs which are associated with bitumen modification as well as helping to alleviate the problem of waste accumulation. In this investigation, a polysulfide polymer derived from waste (waste liquid polysulfide polymer, WLPSP), in pure form and in combination with nanoclay was used to modify bitumen. The amounts of polymer added to the bitumen were 1, 3 and 5 weight percents. In the samples modified with both polymer and nanoclay, the amount of nanoclay was fixed at 2 wt%. The WLPSP polymer not only increased the softening point but also reduced the penetration of bitumen, resulting in improvement of the properties of bitumen at high temperatures. These effects could be observed for all samples of bitumen-polymer-clay and bitumen-nanoclay blends. The bitumen modified with nanoclay-polymer and nanoclay alone showed a further reduction in penetration upon WLPSP modification. The penetration index (PI) of bitumen increased, indicating a lower thermal sensitivity of modified bitumen.